Stood in the fire acheive

As we all know, you get this by being in the lucky zone that Deathwing decides to destroy entirely. My question is, once Deathwing becomes a raid boss, won't everyone wiping on him instantly get this acheivement?

When he becomes a raid boss the achievement will shift into a feat of strength. And by definition, a feat of strength is an achievement that's no longer obtainable. So no, when we raid DW we will not get the achievement. However, we have up until his raid patch to get it (assuming you want it), so just keep doing what you do, and eventually you're bound to run into him.

He's been seen in Uldum frequently on my realm, but I'm sure it's completely random.

- The Hunter's Creed -
"This is my pet. There are many others like him, but this one is mine. He is my best friend. He is my life. I must master him as I master my life.
My pet, without me, is useless. Without my pet, I am useless."

When he becomes a raid boss the achievement will shift into a feat of strength. And by definition, a feat of strength is an achievement that's no longer obtainable. So no, when we raid DW we will not get the achievement. However, we have up until his raid patch to get it (assuming you want it), so just keep doing what you do, and eventually you're bound to run into him.

He's been seen in Uldum frequently on my realm, but I'm sure it's completely random.

It's been reported that Uldum, TH and Southern Barrens are the most frequent areas, at least by my guildies.
I don't think he is restricted to just the new zones, but maybe he's more likely to show up there or other zones that are highly affected.

I see him at least 3 times a week, usually in Uldum, or Twilight Highlands.

I usually say it in guild when it happens, the people waiting around in Orgrimmar can usually use one of the portals to get to the zone to find some fire to stand in. When they changed the achievement so you can get it by standing in it after he leaves the zone made it a lot easier to get.

And by definition, a feat of strength is an achievement that's no longer obtainable. So no, when we raid DW we will not get the achievement.

Feats of Strength tab on WoW tells us this:
Feats of Strength are accomplishments that players will find very difficult if not impossible to earn. Worth no points, Feats of Strength are a collection of past glories on Azeroth.